In one known type of printer, adapted to print on a print medium as it passes over a platen using a plurality of print elements operated selectively by hammers, the print elements 10 are mounted on flexible fingers forming part of a metal band which is in the form of a continuous loop, one print element being mounted on each flexible finger. The print elements extend in a straight line along the band parallel to the longitudinal center line of the band. A bank of hammers extends along the platen and is spaced from the platen so as to define a print region between the hammer bank and the platen. The print medium, such as a continuous web of paper, extends through the print region over the platen so that the hammer bank extends across the width of the print medium. The metal band on which the print elements are mounted also extends through the print region along the platen and across the width of the print medium and is located between the hammer bank and the print medium. An ink ribbon also is located in the print region between the metal band and the print medium.
The metal band is driven continuously past the platen and the hammer bank and across the print medium by a suitable drive system. Operation of any one of the hammers of the bank causes that hammer to move towards the metal band and to strike one of the print elements so as to move that print element on its flexible finger towards the ink ribbon and the print medium. The print element abuts against the ink ribbon, moves the ink ribbon into contact with the print medium and presses the ink ribbon and the print medium against the platen causing the printing of a mark on the print medium in the shape of the print element.
According to one particular printer of the above kind and illustrated by way of example in U.S. Pat. No. 4.428.284. each of the print elements is shaped like a dot and operation of each hammer causes the printing of a dot on the print medium. As the metal band moves continuously across the print medium, operation of selected hammers will result in the printing of a row of dots in positions on the print medium corresponding to the positions of the hammers which are operated. Each hammer is formed with a head having a width in the direction of movement of the band which is greater than the width of a single print element. It is therefore possible for each hammer to print a dot in any position on the print medium which is covered by the hammer by varying the timing of the operation of the hammer relative to the movement of the band. As a result, the dots in the row printed can occupy many selected positions on the print medium. There is only a small gap between each pair of adjacent hammer heads and the hammers can therefore print dots at all required positions along the row being printed.
After one row of dots has been printed the print medium can be moved through a small increment transversely to the length of the platen and the operation can be repeated resulting in the printing of a second row of dots spaced from the first row of dots. By repeating these operations rows of dots can be printed as required.
A character can be printed on the medium by printing dots in selected positions in a matrix, for example a matrix of five columns and seven rows. By printing dots in selected positions in rows as described above and selectively moving the print medium, characters can be printed in selected positions on the medium.
In the printer described above the hammers are mounted together to form a hammer bank which extends along the platen. Each hammer is formed with a head and is associated with a respective actuator which has a finite width. It is desirable for the heads of adjacent hammers to be spaced apart by only a small distance so as to be able to print dots substantially at all positions along a row on the print medium. With such an arrangement each hammer head will cover a plurality of positions in which dots are required to be printed and therefore each hammer will have to be operated a plurality of times in printing a row of dots. In order to reduce the number of times that each hammer will have to be operated in printing a row of dots, the width of each hammer must be reduced. This requires that the width of each hammer and actuator assembly must be reduced.
Various types of printer hammer actuator are known. One particular type with which the present invention is concerned is described by way of example in Canadian Pat. No. 1.135.317. The printer hammer actuator described is an electromagnetic actuator which includes a stator in two halves, each provided with a coil, and a moving armature member which is formed from a non-magnetizable material, for example a synthetic plastic material, and is flat with a rectangular cross section. The armature member is provided with a plurality of armature elements of magnetizable material. The armature elements are spaced apart along the length of the armature member. The armature member is provided with longitudinally extending ribs to add to its strength and to guide it during operation. A hammer head is formed at one end of the armature member.
The stator of the actuator is formed in two halves with pole pieces extending towards each other in pairs and spaced apart so as to form a set of gaps in which the armature member is located. When the armature member is in the rest position each of the armature elements in the armature member is slightly spaced from a respective one of the pairs of pole pieces of the stator. When the coils of the stator are energized, a flux is generated which flows between the pairs of pole pieces and through the armature elements. As a result each armature element is attracted to the adjacent pair of pole pieces and a longitudinal force is exerted on the armature member. The armature member is retained in its rest position by a spring. The longitudinal force causes the armature member to move against the action of this spring and allows the head on the armature to perform a printing operation.